Project Summary Acute respiratory infections (ARI), which generally begin with colonization of the mucosal surfaces of the upper respiratory tract (URT), are a leading cause of morbidity with the highest rate in infants. Both the incidence and duration of URT carriage with common respiratory pathogens declines in adults. Despite substantial evidence linking young age to ARI, identification of age-dependent factors that enhance acquisition and delay clearance of respiratory pathogens have largely been unexplored. Currently, a major obstacle in determining age-dependent mechanisms arises from the exclusive reliance on adult animal modeling. In this study, we will utilize both infant and adult mouse models to determine specific immune mechanisms that are critical for mediating resistance to URT infections by using S. pneumoniae as a model pathogen. Using this model, we found that young age predisposes mice to both acquisition and carriage of S. pneumoniae infection. To study the underlying effects of age we used RNA-sequencing (RNA-Seq) to transcriptionally profile and compare the mucosal epithelia of infant and adult mice at baseline (uninfected). Our preliminary data show an age-dependent alteration of mucosal defense mechanisms, consisting of dampened expression of ubiquitous antimicrobial molecules and impaired interleukin-1 signaling in infant mice compared to adult mice. Furthermore, we observed a significant alteration in central metabolism of the URT indicated by a repression of genes involved in oxidative phosphorylation and peroxisome proliferator activated receptor (PPAR) signaling. These data indicate a potential regulatory role for metabolic pathways of mucosal immune defenses. Given these findings we hypothesize that impaired mucosal defense mechanisms during the neonatal period promote acquisition and stable colonization of URT pathogens, which facilitate bacterial dissemination and promote development of systemic infection. This hypothesis will be addressed by the following Specific Aims: (1) to determine how age of the host impacts URT epithelial barrier defense mechanisms; (2) to characterize the role of interleukin 1 (IL-1) signaling during young age and (3) to elucidate the role of PPAR signaling in mediating epithelial barrier defense. These studies will improve our understanding of age-related differences in mucosal defense mechanisms and facilitate the development of novel therapeutic strategies to protect infants from prevalent URT pathogens.